Example/Datasets/Readme.md link is broken.

[hungry1526]

In this link:https://sql-machine-learning.github.io/sqlflow/doc/installation.html

to "example/datasets/readme.md" is broken.

[wangkuiyi]

Thank you @hungry1526 for pointing this out.

I am filing a sequence of PRs to fix this problem:

1. Better explain running a test-data MySQL container sqlflow#320
2. Refer to README.md by directory name sqlflow#321